The present invention relates to a hollow composite member, and more particularly to a composite spar which utilizes braided fibers at specific angle orientation and a method for facilitating the manufacture thereof.
A rotor blade spar is a primary structural element of a helicopter rotor blade assembly inasmuch as its primary function is to transfer combined flapwise, edgewise, torsional and centrifugal loads to/from a central torque drive hub member. Typically, a leading edge and trailing edge assembly envelop the spar to yield a desired airfoil contour. The spar typically extends along the length of the rotor blade and mounts at its inboard end to a cuff assembly that facilitates mounting to the hub member.
Fiber reinforced resin matrix composite materials are employed due to their advantageous strength to weight ratio. Despite the inherent weight and strength advantages, widespread use thereof has been impeded by the high cost of materials and associated fabrication methods.
Conventional methods for manufacturing composite spars include filament winding and prepreg lay-up of composite material. Although effective, these processes are relatively time and labor intensive.
The prepreg lay-up process is primarily performed by hand. Prepregs have little structural rigidity in their uncured state. The prepregs are hand-stacked and interleaved upon an inflatable mandrel assembly. The lay-up is then transferred into a matched metal mold and autoclave oven cured while the mandrel assembly is inflated to form the finished composite spar. Moreover, prepreg sheets are relatively expensive and require meticulous storage and handling processes which further increase manufacturing expense.
The filament winding process involves a winding process in which the filaments are preimpregnated or the resin is applied while the filaments are wound about a mandrel assembly. The mandrel assembly is of a shape generally corresponding to the required shape of the finished article. The mandrel assembly typically includes a rigid substructure and a bladder disposed over the rigid substructure. The filaments are wound over the mandrel while the mandrel and filaments are reciprocally displaced relative to one another along the longitudinal or winding axis of the mandrel to build a plurality of layers of filamentary material. Upon completion of the filament winding process, the mandrel-wound lay-up is placed in a matched metal mold and cured while the bladder is pressurized to urge the fibers against the matched metal mold.
One disadvantage to filament winding relates to difficulties associated with expanding the fibers against the matched metal mold. As the fibers are initially wound laterally about the mandrel under tension, pressurization sufficient to achieve proper laminate compaction is difficult to achieve. Should the fibers resist complete and uniform compaction, the composite article may become resin-rich or resin-starved in particular areas resulting in poor laminate quality. In the case of an elliptically shaped composite article, the conic regions, i.e., the leading and trailing edges, may be vulnerable to unacceptable variations in fiber volume.
Another disadvantage to conventional filament winding relates to the difficulty associated with establishing fiber orientations at or near 0 degrees relative to the longitudinal axis of the mandrel assembly. Insofar as conventional filament winding apparatus are deficient in this regard, it is common to periodically interrupt the winding operation to interleave unidirectional i.e., 0.degree fibers. Such interleaving operations are laboriously performed by hand and are relatively time consuming which still further increases manufacturing expense.
Accordingly, it is desirable to provide a structurally efficient composite spar which locates fibers in a particular orientation, minimizes weight, and increases damage tolerance. Furthermore, it is desirable to provide an inexpensive manufacturing process which reduces labor intensive process steps, permits ease of material handling, yet maintains exacting quality standards during assembly.